A known circuit for a power supply unit is used, for example in switchgear, to supply electronic overcurrent releases. The current transformer used in this case can be considered to be a non-ideal current source. This means that a secondary current which is proportional to the primary current is not always generated on account of saturation effects.
The power supply unit is used to generate a DC voltage for supplying the electronics of the overcurrent release. In this case, the secondary current which has been rectified by the rectifier charges the charging capacitor, with the result that the voltage rises across the latter. This voltage is monitored by the comparator which switches on the electronic switch when a voltage value predefined by the voltage reference circuit is reached.
This electronic switch which is in the form of a MOSFET, for example, short-circuits the current transformer on the secondary side, with the result that the voltage across the charging capacitor does not rise further. The blocking diode prevents the capacitor from being discharged via the electronic switch. If the voltage across the charging capacitor undershoots a particular lower limit value, the electronic switch is switched off again by the comparator and the charging capacitor is recharged via the secondary current of the current transformer.
This recharging operation can last for a plurality of half-cycles depending on the secondary current in the current transformer. The recharging operation can synchronize with the frequency of the primary current and may result in the current transformer being saturated. This distorts the waveform of the secondary current with respect to that of the primary current. Since the current transformer is often also used to measure the current, small currents, in particular, are measured incorrectly on account of the distortion.